Water adsorption by hydrophobic organic surfaces: Experimental evidence and implications to the atmospheric properties of organic aerosols

The adsorption of water on hydrophobic organized organic thin films, used a s a proxy for atmospheric organic aerosols, was measured simultaneously wit h a molecularly controlled semiconductor resistor (MOCSER) and quartz cryst al microbalance (QCM). Water adsorption was found to be reversible and depe ndent on relative humidity (RH). The MOCSER measurements show that the adso rption kinetics of the first water layer resembles Langmuir behavior. The Q CM findings indicate that adsorption is proportional to the water vapor pre ssure and exceeds the amount equivalent to one monolayer. These results can be explained by the formation of small water clusters on imperfections or structural defects on the organic surface and that the water does not achie ve complete surface coverage even at high relative humidity. This mechanism resembles previous observations of water adsorption to structural defects on inorganic crystals. It is also shown that water can penetrate through th e organic surface and reach the inorganic surface, even with a closely pack ed organic coating. This paper presents applications of these findings to t he properties of organic aerosols.